Electrothermal fluidized bed is the basis for the high-temperature heat engineering processes development ( First part)

 

One of the main advantages of the high-temperature chemistry technological processes implementation (iron oxides direct reduction, carbides production, carbon materials enrichment, coal gasification and activation, etc.) in fluidized bed units is the heat and mass transfer intensification possibility and highly productive continuous technology creation [1 -2].


High-temperature processes occur in the temperature range of 1000 - 3000˚С. Such processes require a constant supply of heat energy, which compensates for heat losses and maintains the required temperature. The source of heat energy can be:

  • hot gaseous coolant, which is fed into the furnace as a fluidizing agent;
  • burning fuel (solid or gaseous) directly in the bed;
  • electric heating.

 The supply of hot combustion products to the bed and combustion of the fuel directly in the bed ensure the operation of the furnaces at temperatures up to 900-1200 ° C [2-4]. Based on this knowledge, the main method of high-temperature processing in a fluidized bed is the use of electric heating, which includes several areas:

  •  induction heating of electrically conductive particles of the fluidized bed;
  •  plasma heating, indirect heating by electric heaters;
  •  heating by passing a current through the fluidized bed (electrothermal fluidized bed).

The electrothermal fluidized bed technology ensures the operation of furnaces at bed temperatures in the range from 1000 ° C to 3000 ° C. The material processing time is unlimited and allows you to organize a continuous production process.
The basic principle of creating an electrothermal fluidized bed is the presence of three necessary components (Fig. 1):

  •  the presence of at least two electrodes;
  •  fluidized bed with electrically conductive material;
  •  current source.

The choice of the fluidized bed implementation is determined by the process temperature level, which determines the using refractory materials possibility for the furnace working chamber manufacture. In this regard, all technological processes and furnace designs are divided for:

  •  “low temperature” - layer temperature 1000 - 1600 ˚С;
  •  “high temperature” - layer temperature 1600 — 3000˚С.

 Options for the implementation of electrothermal fluidized bed

Fig. 1 Options for the implementation of electrothermal fluidized bed [1]

1 - current source, 2 - housing with thermal insulation, 3 - fluidized bed, 4 - electrode,
5 - gas distribution grid - electrode, 6 - gas distribution grid,
7 - inert gas supply, 8 - removal of gases from the furnace, 9 - non-conductive lining of the furnace working chamber, 10 - electroconductive lining of the furnace working chamber.

Various refractory materials are used for the production of "low-temperature" furnaces”[5, 6]: chamotte, corundum, magnesia, dinas. But, only graphite is used for “high-temperature” furnaces, which determines the furnace design and technological process formation.

 

"Low-temperature" electrothermal fluidized bed furnace

"Low-temperature" processes with an electrothermal fluidized bed include: direct reduction of iron using solid carbon material as a reducing agent [7], production of titanium carbide TiC [8], silicon carbide SiC [9,10] and zirconium carbide [9], steam gasification of coke [12], heat treatment of metal products [13], production of zirconium chloride ZrCl4 [14], production of hydrogen by hydrocarbon pyrolysis [15], encapsulation of quartz sand with pyrocarbon [16], calcination of green petroleum coke [17].


We can divide all designs of “low temperature” electrothermal fluidized bed furnaces in the electric current direction in the fluidized bed: across the layer (in the horizontal plane) and along the layer axis (in the vertical plane). Figure 1a, 2, and 3 show furnace designs with two electrodes [17, 18] that are immersed in a fluidized bed of electrically conductive particles, which ensures that the layer heats up when current flows from one electrode to another in the horizontal plane. A similar furnace design is proposed by the author of the patent [19].

 

Electrothermal fluidized bed furnace for calcining petroleum coke
Fig. 2 Electrothermal fluidized bed furnace for calcining petroleum coke [17]

 

Electrothermal fluidized bed furnace for carrying out catalytic reactions

Fig.3 Electrothermal fluidized bed furnace for carrying out catalytic reactions [18]

1 - electrodes, 2 - fluidized bed, 3 - gas distribution grid, 4, 5 - supply and evacuation of inert gas, 6 - heat protection shields.

 

Electrothermal fluidized bed furnace

Fig.4 Electrothermal fluidized bed furnace [22]

1- furnace stove, 2 - electrodes, 3 - fluidized bed, 4 - gas distribution grid.


The second design version with the electric current transverse movement is a furnace with a cylindrical working chamber, while its body is an electrode. The second electrode is located in the center of the working chamber. Thus, the current moves in the radial direction [7, 10, 12, 14, 20]. An example of such a furnace for the direct reduction of iron oxides is presented in Figure 5 [7].

 

Furnace for the direct reduction of iron oxides


Fig.5 Furnace for the direct reduction of iron oxides [7]
1 - central electrode, 2 - lined furnace body, 3 - side electrode, 4 - inert gas supply, 5 - reaction gas supply, 6 - chilled material discharge feeder.

There are a number of electrothermal fluidized bed furnaces designs with vertical current movement [21-24]. The upper electrode significantly overlaps the furnace working chamber and has a developed surface shape (crown). The gas distribution grid is used as the bottom electrode (Fig.6.7).

 

Electrothermal fluidized bed furnace for the pyrolysis of hydrocarbons

Fig. 6 Electrothermal fluidized bed furnace for the pyrolysis of hydrocarbons [21].

1- thermal insulation, 2- furnace body, 3- working chamber made of refractory material, 4 - upper electrode, 5 - current leads, 6 - supply of hydrocarbons, 7 - supply of carbon material to the layer, 8 - discharge of carbon material from the working chamber, 9 - gas distribution grid - the bottom electrode, 10 - removal of gas from the working chamber.

 

Electrothermal fluidized bed furnace for encapsulation of quartz sand with pyrocarbon


Fig.7 Electrothermal fluidized bed furnace for encapsulation of quartz sand with pyrocarbon [22].

1- Security external heater, 2,3- top movable electrode, 4 - gas distribution grid - the bottom electrode,
5- supply of hydrocarbon gases.

 

The design of the furnace with lattice electrodes suggests the placement of electrodes along the height of the fluidized bed, and between them there is a refractory nozzle [23] (Fig. 8). Thus, the constrained electrothermal fluidized bed is realized. The electrodes are alternately connected to different phases.

Horseshoe current movement is implemented in the furnace design [25] (Fig. 9). The electrodes are the individual elements of the gas distribution grid, which are separated by electrical insulating inserts in the partitions form.


"High-temperature" electrothermal fluidized bed furnaces

"High-temperature" electrothermal fluidized bed furnaces have a design similar to [7, 10, 12, 14, 20] with a central electrode and an electrically conductive working chamber with a lining, which is the second electrode. The working chamber is made of graphite materials and inert gases such as argon and nitrogen are used as a fluidizing agent.

High-temperature furnaces operate in a continuous mode with constant loading of raw materials and unloading of the processed material [25-28].
A typical design of a high-temperature furnace is presented at Figure 10.

 

Сonstrained electrothermal layer furnace

Fig.8 Сonstrained electrothermal layer furnace [23].

1- housing, 2- gas distribution grid, 3,4 - electrodes, 5,6 –current rods, 7 - electrical insulating bushes.

 

Electrothermal fluidized bed furnace

Fig.9 Electrothermal fluidized bed furnace [24]

1 - furnace lining, 2 - gas distribution grid section - electrode,
3 - vertical partition of dielectric, 4 - current leads, 5 - fluidized bed.

 

High-temperature electrothermal fluidized bed furnace for processing carbon material


Fig. 10 High-temperature electrothermal fluidized bed furnace for processing carbon material

1 - centralite graphite electrode, 2 - supply of raw material to the furnace,
3 - furnace body, 4 - thermal insulation - graphite lining of the working chamber, 6 - supply of inert gas, 7 - distribution chamber of the processed material, 8 - gas distribution grid, 9 - removal of exhaust gases, 10 - the first stage refrigerator for processed material, 2 - the second stage refrigerator for processed material.

 

It is necessary to make a structural choice for the following elements to develop a design of electrothermal fluidized bed furnaces:

  •  furnace working chamber;
  •  electrodes;
  •  gas distribution grid;
  •  removal of the processed material;
  •  loading of raw material;
  •  removal of exhaust gases;
  •  cooling of the processed product.

The second part of the article will be devoted to the analysis and recommendations on the choice of designs of these furnace elements.

On our website https://tmec.com.ua you will find more information.

 

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by Fedorov S.S., Sybir A.V., Hubynskyi S.M., Hubynskyi M.V., Gogotsi A.G.

April 11, 2019